Once I fought the breeder on the route that takes you to the Desert Resort and Nimbasa City,I used my Poke Dream Radar Dialga and I used Slash,the Marill was a few levels lower than Dialga (Which was level 24 at that time) and it survived!It didn't get down to half!It's confusing.Please respond.

Anyway back to the reason why you did so little damage. At base 70 power, Slash doesn't exactly measure up as the strongest physical attack ever. Additionally, it also depends on your stats - for example, if your Dialga has low attack IVs/EVs, it's attack stat will be lower, making it do less damage to Marill.

Anyway recheck your Dialga's attack stat. And oh yeah, its nature, as if it has an Attack hindering nature, that reduces it's attack stat by a further 10%, making it even weaker.

Since when has Dialga's Attack Stat been "not supposed to be used"? Slap on an Outrage and you can kill half the game in one hit. Also all your move suggestions on the last line will miss 30% of the time. If you are to suggest any moves for ingame, at least suggest moves which have good PP and reliability as well as power. Marill isn't as defensive as you think, btw.